Online defaults to order:hot, with an Account toggle

New online_hot_default preference (on by default, so guests and accounts
that never saved preferences get it): opening /online without a search
replaces the URL with ?tags=order:hot — e621's metatag for the order the
Hot page uses — so it is visible in the search field and shareable.
Existing searches are never touched: they live in the URL, so a refresh
or back/forward keeps them, while a fresh visit (nav pill, first time,
after a long time away) gets the hot default again. Turning the toggle
off opens Online on the site-wide newest posts as before.

The toggle sits in Account -> Browsing preferences and saves with the
rest of the settings; the backend validates the new boolean
(400 for non-boolean input).

frontend/src/features/account/PreferencesCard.tsx

@@ -170,6 +170,28 @@ export function PreferencesCard() {
           />
         </label>
 
+        <label className="flex items-start gap-2 text-sm text-ctp-subtext0">
+          <input
+            type="checkbox"
+            checked={values.online_hot_default ?? true}
+            onChange={(event) =>
+              setValues((current) => ({
+                ...current,
+                online_hot_default: event.target.checked,
+              }))
+            }
+            className="mt-0.5 accent-ctp-mauve"
+          />
+          <span>
+            Online starts with{" "}
+            <span className="font-mono text-ctp-blue">order:hot</span>
+            <span className="mt-0.5 block text-[11px] leading-relaxed text-ctp-overlay0">
+              Used when you open Online without a search. With it off, Online
+              opens on the site-wide newest posts.
+            </span>
+          </span>
+        </label>
+
         <div>
           <Button onClick={() => void handleSave()} disabled={saving}>
             {saving ? "Saving…" : "Save preferences"}

frontend/src/features/online/OnlinePage.tsx

@@ -1,6 +1,6 @@
 import { keepPreviousData, useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { X } from "lucide-react";
-import { useMemo, useState } from "react";
+import { useEffect, useMemo, useRef, useState } from "react";
 import { Link, useLocation, useSearchParams } from "react-router-dom";
 
 import { CollapsibleSidebar } from "@/components/CollapsibleSidebar";
@@ -18,6 +18,7 @@ import {
   type E621Post,
 } from "@/lib/e621";
 import { useE621 } from "@/store/e621";
+import { useAuth } from "@/store/auth";
 import { useUi } from "@/store/ui";
 import { toast } from "@/store/toasts";
 
@@ -34,11 +35,24 @@ export default function OnlinePage() {
   const credentials = useE621((state) => state.credentials);
   const blacklist = useE621((state) => state.blacklist);
   const saveBlacklist = useE621((state) => state.saveBlacklist);
+  const user = useAuth((state) => state.user);
+  const authReady = useAuth((state) => state.ready);
   const queryClient = useQueryClient();
   const [draft, setDraft] = useState<string | null>(null);
   const [blacklistDraft, setBlacklistDraft] = useState("");
   const tagInput = draft ?? tags;
   const zoom = useUi((state) => state.zoom);
+  const appliedHotDefault = useRef(false);
+
+  // Landing on Online without a search uses the order:hot default (on unless
+  // the account turns it off). Existing searches are left alone.
+  useEffect(() => {
+    if (!authReady || appliedHotDefault.current) return;
+    appliedHotDefault.current = true;
+    if (tags) return;
+    if (user?.preferences?.online_hot_default === false) return;
+    setSearchParams(buildParams("order:hot", 1), { replace: true });
+  }, [authReady, tags, user, setSearchParams]);
 
   const blacklistMutation = useMutation({
     mutationFn: (lines: string[]) => saveBlacklist(lines),

frontend/src/lib/types.ts

@@ -4,6 +4,7 @@ export interface UserPreferences {
   ordering?: string;
   per_page?: number;
   zoom?: number;
+  online_hot_default?: boolean;
 }
 
 export interface User {